The mechanism of insulin's vasomotor effect in the skeletal muscle circulation is unclear. The purpose of this study was to examine whether the hemodynamic response to insulin involves the concurrent release of endothelin (ET) and nitric oxide (NO), two substances with opposing vasoactive properties. BQ-123 (100 nmol/min), a selective ETA blocker, alone or in combination with BQ-788 (50 nmol/min), a selective ETB blocker, was given into the brachial artery of 7 healthy subjects during local infusion of saline or insulin (0.1 mU/kg/min). During saline, either selective ETA or nonselective ET blockade did not result in any significant change in forearm blood flow (FBF; strain-gauge plethysmography) from baseline (both P<0.05). Insulin administration resulted in increased local hormone's concentrations (P<0.001), but did not modify FBF (3.21+/-0.37 mL/min/dL before and 3.12+/-0.43 mL/min/dL after 2 hours of infusion; P<0.05) During hyperinsulinemia, both selective and nonselective ET antagonism resulted in a significant increase in FBF from baseline (72% and 64%, respectively, after 2 hours; both P<0.001). Superimposing L-NMMA (4 umol/min for 30 min) to nonselective ET blockade did not result in any significant change in FBF during saline (2.81+/-0.26 mL/min/dL before and 2.43+/-0.21 mL/min/dL after L-NMMA; P<0.05), but was associated with a significant decrease in FBF during hyperinsulinemia (from 4.41+/-0.67 to 2.82+/-0.31 mL/min/dL; P<0.05), indicating that the proportion of NO-dependent FBF was 12% during saline compared to 32% during hyperinsulinemia (P<0.05). Our findings indicate that insulin stimulates both ET and NO activity in the forearm circulation of healthy humans. An imbalance between endothelial release of these two substances might be involved in the athogenesis of hypertension and atherosclerosis in hyperinsulinemic states.